This invention relates to a method of controlling a stepping motor used to perform a line feed in a printer and a driver circuit for driving such a printer. Such a stepping motor is called below a line feed motor.
Printers such as those used in office automation equipment are furnished with a line-feed motor for automatic performance of the line-feed operation. When a line feed is to be made, the line-feed motor receives, on command from the host controller, pulses to cause it to rotate through an angle corresponding to the distance the medium is to be fed. Driven by these pulses, the motor rotates and thereby feeds the printing medium by the commanded amount. After the line-feed operation, the line-feed motor is held by a certain holding torque, which is selected so as to prevent the printing medium from being moved by outside forces, but to permit the printing medium to be fed manually by the operator. This torque is on the order of 1/5 to 1/10 of the torque used to feed the printing medium, the exact value depending on the particular line-feed motor.
A block diagram of the printer's circuit configuration is shown in FIG. 1. The microprocessor (.mu.CPU) 1 communicates via an address/data bus 2 with a read-only memory (ROM) 3 and a random-access memory (RAM) 4 which it uses to control the printer's operation. The driver circuit 5 causes the line-feed motor 6 to rotate or to hold. The interface controller 7 receives commands from a host controller and sends printer information to the host via the interface connector 8.
FIG. 2 shows a specific example of the circuit configuration of the driver 5 in FIG. 1. This circuit is well known, so a detailed explanation is omitted. It comprises a register 5a which receives timing signals from the microprocessor 1 and produces control signals OVDV, .phi.1, .phi.2, .phi.3 and .phi.4. These control signals are supplied through open-collector drivers D0 and D1 to D4 to the bases of transistors T0a and T1 to T4a whose emitters are grounded. The collector of the transistor T0a is coupled to the base of an additional transistor T0b whose emitter is connected to a over-voltage drive power supply Vp(e.g., 38 V). The collector of the transistor T0b is connected to center taps of stator windings 6a, 6b of line-feed motor 6. The ends of the windings 6a, 6b are connected to the collectors of the transistors T1 to T4. The center taps of the windings 6a, 6b are also coupled through a diode 10 and a resistor 9 to the logic-level power supply Vcc.
At the beginning of each cycle for each step, the open-collector driver D0, the transistors T0a and T0b are made conductive by the control signal OVDV, and the over-drive power supply Vp is connected to the stator windings through the transistors T0b. When the transistor T0b is nonconductive, the power supply Vcc of a lower voltage supplies current to the stator windings 6a, 6b. This current produces holding torque when periodic and sequential alternation of the signals .phi.1 to .phi.4 are terminated.
In the prior art shown in FIGS. 1 and 2 the printer operates as follows. When the interface controller 7 receives print data from the host via the interface connector 8, the microprocessor 1 first stores the data in the RAM 4. When the data for a line feed (or carriage return) are received, the printer prints one line of data by means of a print head and spacing mechanism not shown in the drawings. Then, if a line feed is to be performed, the signals .phi.1 to .phi.4 are alternated in sequence. The overdrive voltage Vp is applied in the first part of each cycle for each step. In the latter part of each cycle for each step, a lower voltage Vcc is applied. The voltages are used for rotating the line-feed motor 6. Thus the line-feed motor 6 is driven to feed the printing medium by the predetermined amount.
After the line-feed operation, the alternation of the signals .phi.1 to .phi.4 is terminated and the states of the signals .phi.1 to .phi.4 at the time of the termination of the alternation of the signals .phi.1 to .phi.4 are maintained. That is, two of the signals .phi.1 to .phi.4 that are at the high level at the time the alternation is terminated are kept high and the other two of the signals .phi.1 to .phi.4 that are at the low level at the time the alternation is terminated are kept low. As a result, a holding torque is generated in the line-feed motor 6 by the current which flows from the power supply Vcc through the resistor 9 and the diode. The holding torque prevents the printing medium from being moved by external forces.
In the line-feed motor control method described above, a problem arises because the motor has only two torques: the small holding torque that permits the operator to perform manual feeding, and the large torque by which the line-feed motor performs the feeding operation. During the time between one line-feed operation and the next (while the printer is spacing and printing), friction and other large load fluctuations that arise may overcome the small holding torque, leading to step-out.